1. Field of the Invention
The present invention generally relates to a wavelength multiplexing method, and an apparatus thereof, and especially relates to the wavelength multiplexing method of a WDM (Wavelength Division Multiplexing) optical transmission system, and the apparatus thereof.
2. Description of the Related Art
In WDM optical transmission systems, transmission characteristics are heavily reliant upon power and optical SNR (signal to noise ratio) of each of the wavelengths, serving as optical channels, which are multiplexed. For this reason, the power levels of all the wavelengths are required to be equal to each other. To achieve this, each wavelength is input into a variable optical attenuator (VAT), and the power levels of the input wavelengths are made equal to each other by attenuating the power (for example, patent reference 1 listed below is referred to).
FIG. 1 shows a block diagram of an example of a conventional wavelength multiplexing apparatus (WDM apparatus). In FIG. 1, wavelengths CH1 through CHn are supplied to variable optical attenuators 101 through 10n, respectively, and are multiplexed by a multiplexing unit (MUX) 12. The multiplexed wavelengths constitute an optical signal, which is amplified by a transmitting amplifier (AMP) 14, and is output to an optical transmission path.
A part of the optical signal output from the transmitting amplifier 14 is branched (split off) at an optical coupler 16, and the branched part of the optical signal is supplied to an optical monitor unit 18. The optical monitor unit 18 performs an optical spectrum analysis with an optical spectrum analyzer (SAU), calculates an attenuation amount to be provided by each of the variable optical attenuators 101 through 10n, and controls the variable optical attenuators 101 through 10n via a serial interface 19 such that the optical power levels of all the wavelengths become equal.
Patent Reference 1
Japanese Provisional Patent No. H9-261205.
The above-mentioned conventional method is mainly for a point-to-point system, among various configurations of WDM systems, as shown at (A) of FIG. 2, where an input wavelength is provided by a stable optical source, such as a transponder. For this reason, the conventional method poses certain problems in a WDM system that is configured differently, namely, in the case of an add/drop apparatus 20 wherein a wavelength (channel) is branched (dropped) and another wavelength is inserted (added) as shown at (B) of FIG. 2, and a cross connect apparatus 22 that performs an optical switching function in units of wavelengths as shown at (C) of FIG. 2.
One of the conventional problems (the first problem) is relative to feedback speed, which is a function of operational performances of the optical spectrum analyzer and the variable optical attenuators. The optical spectrum analyzer monitors all the wavelengths that are multiplexed. In a typical monitoring method, a part of the optical signal is branched, and used for monitoring. The branched part of the optical signal is input to a diffraction lattice so that each wavelength is separated. Then, signal strength of each wavelength is measured by a sweeping operation.
FIG. 3 shows a conventional feedback control method. At Step S10, all the wavelengths are monitored by the optical spectrum analyzer SAU on the left-hand side of FIG. 3. At Step S11, a required attenuation amount of each wavelength is calculated based on variation among the wavelength levels. At Step S12, the required attenuation amount is transmitted to the respective variable optical attenuators VAT of each wavelength via the serial interface. Then, with reference to the right-hand side of FIG. 3, the required attenuation amount is received by the respective VAT at Step S13, and the respective attenuation amount of the VAT is changed to the required attenuation amount at Step S14. Consequently, at Step S15, the output level of the VAT is changed, and the output level of the corresponding wavelength (channel) is changed.
As for the signal strength of each wavelength being measured, in order to obtain an accurate measurement under this conventional method, the interval between measurements is required to be longer than 100 ms. Further, the serial interface, which is normally used for transmitting the required attenuation amount of each wavelength to the respective variable optical attenuator from the optical spectrum analyzer, is required to handle a large number of wavelengths, which is of a magnitude of 100, and growing.
Even if the measurement speed of the optical spectrum analyzer is improved by simultaneously monitoring all the wavelengths, there is a limit to improvement of the transmission speed of the serial interface. For this reason, the feedback cycle cannot be raised to a desired high speed by the conventional method, wherein only the optical spectrum analyzer directly controls the required attenuation amount of the variable optical attenuators.
When the input level of an optical channel fluctuates, the conventional control method can respond to a slow change over a period longer than 100 ms, and such a slow change can be absorbed (dealt with). However, if the input level fluctuation is sudden over a period shorter than 100 ms, the fluctuation cannot be absorbed by the conventional method, and the fluctuation is passed on to a next stage as it is.
In the most fundamental WDM configuration, i.e., in the point-to-point configuration, all wavelengths (channels) are input from stable optical sources, such as a transponder, and the probability of the above sudden fluctuation occurring is low. On the other hand, in the case of the add/drop apparatus and the cross connect apparatus, an input wavelength is a signal that is individually branched from an upstream WDM apparatus. For this reason, a sudden, and often big fluctuation (power level change) may occur in the multiplexed optical signal. Unless a countermeasure is provided, the sudden and big level change falls out to a downstream WDM stage, which is the above first problem.
In the case of the add/drop apparatus and the cross connect apparatus, after the multiplexed optical signal is demultiplexed, some or all of the wavelengths are dropped or switched, and the switched wavelengths are multiplexed again, and transmitted. When multiplexing, it is necessary to make the transmitting level of all wavelengths uniform like the point-to-point configuration, requiring the feedback control function using the variable optical attenuator and the optical spectrum analyzer.
Major differences of the add/drop apparatus and the cross connect apparatus compared to the point-to-point configuration are sudden and great variations of the optical input level, which are often caused by change of the upstream output level due to incrementing and decrementing of the wavelengths of an upstream WDM apparatus (since the input to the multiplexing unit is the output of the upstream WDM apparatus), fall of the level due to failure of the optical amplifier, an instantaneous disconnection of the optical signal when changing a path at the optical switch, and an automatic back-off to a safe optical power level when an optical fiber is accidentally disconnected.
By the conventional control method, wherein the feedback speed is low, the sudden level change cannot be absorbed, but is passed on to a downstream WDM apparatus. Accordingly, where the WDM system is configured with multiple stages of add/drop apparatuses and cross connect apparatuses, the level changes are rapidly accumulated and passed downstream, causing receiver damage and signal error. This restricts the number of the stages of add/drop apparatuses and cross connect apparatuses.
Further, an automatic level control (ALC) is often provided for regulating the output power, which poses a risk of the ALC amplifying the sudden and great level change. This restricts the number of repeaters, and, therefore, the transmission distance. As the result, the performance of the transmission system is degraded.
The second problem is related to detection of an input signal that contains an ASE (amplified spontaneous emission). While an ASE is not contained in an optical signal from a stable optical source, as is the case of a point-to-point system, an ASE is contained in an optical signal in the case of the add/drop apparatus and the cross connect apparatus. This is because an ASE is generated when the optical signal passes through the optical amplifier in the upstream WDM apparatus. For this reason, in the case of the add/drop apparatus and the cross connect apparatus, the optical signal input to the VAT (variable optical attenuator) contains an ASE.
A threshold detection level alarm is set up at the VAT for detecting a disconnection and a level decline of the optical signal such that management and triggering of incrementing/decrementing of a wavelength are carried out. At this juncture, if an ASE is contained in the optical signal, detection may become inaccurate, for example, disconnection and level decline in the upstream WDM apparatus are not detected by the downstream VAT.
Consequently, incorrect operations take place, such as a proper optical signal being disconnected, and an optical signal containing only an ASE being transmitted to later stages.
The third problem is related to the dynamic range of the optical signal input to the variable optical attenuator VAT. An optical signal supplied from a stable optical source is relatively well regulated, varying within a range of about ±2 dB under normal operating conditions of, e.g., 0 dBm. On the other hand, in the case of the add/drop apparatus and the cross connect apparatus, an input signal is provided by the upstream WDM apparatus, where various factors cause great variation, which can range ±10 dB. The factors include a loss variation due to a tilting status (explained below) of the upstream WDM apparatus, a loss variation due to path differences relative to branching and adding, and a loss variation due to an increased number of optical connectors.
Although a variable optical attenuator providing a dynamic range of about 30 dB is available, its guaranteed input-and-output linearity range is limited to, e.g., about 10 dB or less. Accordingly, a highly precise level stabilization control with accuracy in the order of 0.1 dB, which is required by WDM, is difficult to obtain by the conventional method.